In the manufacture of integrated circuits, very thin lines or holes down to 100 nm or sometimes smaller are patterned into photo resist and then often transferred using an etching process into a layer of material below on a silicon wafer. It is extremely important to inspect and control the width and profile (also known as critical dimensions or CDs) of these lines or holes. Traditionally the inspection of CDs that are smaller than the wavelength of visible light has been done using large and expensive scanning electron microscopes. In many cases, however, manufacturers would like to have measurements immediately after the photoresist has been patterned or etched to have tight control of the process before it drifts out of spec. Testing the wafer early during production and controlling the fabrication steps according to the test results helps to keep production costs low and to keep yields high. Ideally the measurement tool would be integrated into the wafer track that develops the photoresist or integrated into the wafer etching tool.
In typical stand-alone instruments, the wafer is moved on a stage, while the measurement optics remain stationary. Also, when the angle of incidence on the wafer is other than zero (e.g. in ellipsometers), the wafer is preferably oriented so that the plane of incidence is perpendicular to the lines on the wafer.
An integrated CD measurement tool must be both fast and compact, and must not damage the wafer under test. The size constraints usually mean that the wafer can not be translated across its full diameter in 2 horizontal axes to measure different sites on the wafer. Hence, a portion of the test instrument must move in one or more axes to cover the wafer. The wafer might also rotate, but this is less desirable in systems without full X-Y movement that have a preferred measurement orientation with respect to certain wafer features. Furthermore, some wafer processing tools into which the present invention may be integrated require that the wafer not move so that the processing tool robot can pick up the wafer at any time. The wafer may also be loaded into the measurement tool at an arbitrary angle creating further complications for instruments that have a preferred measurement orientation with respect to certain wafer features.
One general technique that has promise for integrated CD measurements is scatterometry. This technique takes advantage of the fact that an array of small lines or holes affects the properties of the light in the zero order that is reflected (or, for transparent samples, transmitted) from such an array. Various measurable properties of the zero-order light will vary depending on the dimensions of the structure on the wafer. Often such parameters are measured versus wavelength, and in some cases, versus angle of incidence on the sample. Normal-incidence spectroscopic reflectometers show particular promise because they can be used with the wafers in any arbitrary orientation.
Typically, CD measurements have been made using instruments such as ellipsometers or reflectometers that were originally designed to measure film thickness. The data from such instruments is usually fed to a processor, which analyzes the measurements, usually by accessing a library of theoretically generated data for a range of array dimensions and film properties near those of the expected dimensions of the sample.
The measured data are compared to the library and a best fit match to a data set in the library is found. The processor then outputs the corresponding dimensions.
Since there are multiple independent unknown variables that may need to be measured, such as line width, line edge slope, top film thickness, underlying film thickness, or film refractive index, it is desirable that the measurement technique measure as many multiple independent parameters as is practical.
Coulombe et al. (‘Ellipsometric-Scatterometry for sub-0.1 m CD measurements,’ SPIE, Vol. 3332, p. 282-293) investigated reflectometry and ellipsometry of line gratings as a function of angle of incidence and azimuth.
One object of the present invention was to create a scatterometer for measuring CDs and possibly overlay error on periodic structures that is compact and well suited for integration into a wafer process tool.
Another object was to be able to measure on structures at different azimuth orientations.
Another object was to be able to collect as much independent data as practical from the sample. Another object was to be able to measure structures 100 nm wide or smaller.
Another object of the invention was to be able to measure structures at the optimal azimuth angle or angles regardless of the azimuth orientation of the sample.